Inductively coupled plasmas (ICP) are routinely used in a number of different applications including materials processing, production of activated gases, pollutant abatement and many others. In such devices, a coil is placed in close proximity to, around or within a vacuum chamber. When the coil is excited with radio frequency (RF), the electromagnetic fields induced around the coil create and sustain a gas plasma discharge within the vacuum apparatus. The plasma is coupled to the coil either through the air or through a magnetic core. In the latter case, the sources are called transformer coupled plasma (TCP) sources. The power supply used to excite the coil is usually composed of a direct current (DC) section followed by an RF section.
Often, it is desirable to know the amount of RF power delivered to the plasma discharge. In order to calculate the amount of power delivered, voltage and current values, as well as the phase angle between the voltage and current, must be measured. Typically, these measurements are effected directly on the plasma load.
Nevertheless, in some TCP applications voltage and current measurements are taken at the output of the DC section of the power supply. Although this methodology makes both the measurement and the calculation easier, the value of the power obtained is not an accurate estimate of the power delivered to the plasma due to losses in the switches of the RF section and other lossy components in the circuit located between the output of the DC section and the plasma load.
Alternatively, some applications implement RF measurements at some point between the output of the power supply and the TCP reactor. Due to the time variation of the signals, these measurements are generally not as precise as the measurements effected at the input of the DC section of the power supply. The advantage of this type of measurement is that they exclude the losses attributed to the components in the RF power generator. However, other losses continue to be present. Thus, accurate values of the power delivered to the plasma are difficult to obtain by measuring current and voltage at the output of the RF power supply.
Therefore, a mechanism to provide a direct estimation of the power delivered to the plasma discharge is desired.
The invention relates to a method for providing a direct estimation of the power delivered to the plasma in a TCP source. According to one embodiment, an apparatus is described. The apparatus includes a vacuum chamber and an electrical transformer to induce an electromagnetic field within the vacuum chamber. The transformer includes a primary winding, a secondary winding formed by the plasma loop, and a separate secondary winding implemented to measure the voltage along the plasma loop. The apparatus also includes a current transformer to measure the current flow through the plasma loop.